1. Field of the Invention
The present invention relates in general to reactors for treating substrates. More particularly, the invention relates to reactors that process wafers and a method of operating such a reactor.
2. Description of the Related Art
When processing a substrate, such as a semiconductor wafer, the substrate is often heated with a heating means within a body of a furnace. It is customary that the energy supply to the heating means is controlled such that the measured temperature of the furnace body is substantially constant and has a desired value. When a number of substrates are subjected to a heat treatment one after the other, heat is withdrawn from the furnace body on the side of the furnace body adjacent to the substrate. Over the course of time a fall in the temperature of the furnace body will be detected by the temperature sensor. As a response to this, the energy supply to the heating means will be increased to such an extent that the furnace body again reaches the desired temperature. In view of the relatively high thermal capacity of the furnace body, this is a process that proceeds slowly and it can be some time before stable conditions have been established, in particular in those cases where the thermal capacity of the furnace body is so high and the treatment time so short that the temperature of the furnace body has still not been restored at the end of the treatment of a substrate. When a subsequent substrate has been loaded, heat is again withdrawn from the furnace body and in this way the temperature deviation can become increasingly greater for an initial number of substrates to be treated before it is finally restored as a result of the slow progression of the control process. The substrates subjected to treatment during this period will have received a non-uniform heat treatment.
These differences in heat treatment can be even greater than appears from the measured values produced by the temperature sensor. The temperature sensor is usually located within the furnace body, some distance away from the surface of the furnace body that is adjacent to the substrate. The heat, on the other hand, is withdrawn via the surface of the furnace body adjacent to the substrate. Decreases in the temperature of the furnace body of 10° C. or more are possible at the surface or in the immediate vicinity of the surface. This is, of course, undesirable.
In the case of some heat treatments according to the prior art, after the substrate has been positioned in the vicinity of the furnace body, there is a waiting period until a stable, desired temperature has been established, after which the actual treatment, for example the deposition of a layer with the aid of plasma enhanced chemical vapor deposition, starts. However, in some heat treatments (e.g., annealing), the entire temperature-time-profile (i.e., the “thermal budget”) plays an important role in the treatment, especially when the treatment temperature is higher than approximately 500° C. Imposing a particular thermal budget on the substrates can even be the sole purpose of the treatment (e.g., without the formation of a layer on the substrate during the treatment). In such cases controlled and reproducible heating of the substrates is just as important as the final treatment temperature. In other words, it is important to achieve a thermal budget that is identical for all substrates when subjecting substrates to heat treatment. In principle, positioning the substrates in the vicinity of a relatively massive, heated furnace body is an extremely suitable method for this purpose, provided the disadvantages described above can be avoided.